Safety and performance are important considerations in a drilling riser. With trends over the past decades to exploit resources in deeper waters and harsher environments, ensuring the safety and performance of drilling risers has become a challenging task.
A riser tensioning system aims to compensate for relative motions between a floating drilling rig and the seabed, which are joined by a rigid riser string. In conventional systems, the most widely used riser tensioning system is a hydro-pneumatic riser tensioning system consisting of hydro-pneumatic cylinders, air/oil accumulators, and air pressure vessels. However, there are short-comings in hydro-pneumatic tensioning systems.
First, the response time for a hydro-pneumatic tensioning system is too slow for certain situations. The relatively slow operation of pneumatic systems results in a long control response time, which is the time between issuing a command and force being applied by the tension system. In certain situations, such as during an emergency riser disconnect, the tension changing response may be too slow. The slow, large over-pulling force may accelerate free riser pipes outward, allowing them to jump out, and consequently damage the drilling rig floor and riser pipes.
Second, increasing longitudinal over-pull tension, the conventional method in hydro-pneumatic tensioning systems used to suppress destructive vortex-induced vibration (VIV), causes stress on the supporting equipment, increases wear and tear on the tensioning system, and increases riser pipe fatigue. Furthermore, increasing longitudinal over-pull tension raises safety concerns in situations where a pair of hydro-pneumatic tensioners are receiving maintenance while the drilling rig is experiencing high wave conditions.
Third, a hydro-pneumatic tensioning system is a relatively complex and costly system that requires a significant amount of maintenance and is at risk for hydraulic fluid leakage. A hydro-pneumatic tensioning system includes a hydro-pneumatic cylinder rod and a seal that are exposed to bending due to factors such as vortex-induced vibration (VIV) or unequal and non-linear loading caused by vessel roll and pitch. These factors may cause high failure risk and may require a high maintenance cost to avoid hydraulic fluid leakage and risks of environmental pollution. Furthermore, the complex hydro-pneumatic system includes a significant volume of air accumulators and reservoirs that consume useful floor space on a drilling rig.
One important function of the tensioners is to control the drilling riser. The drilling riser is the connection between a platform at a water surface, such as a drilling vessel, and a subsea blowout preventer (BOP). The drilling riser circulates mud and cuttings and is an outer protection system for the drilling pipes and drill bit. Due to extreme handling, or harsh environmental conditions, a planned or emergency riser disconnect is probable. During this event, the riser is lifted up by the riser tensioning system. The elastic energy stored in the long riser string is released, and the riser “recoils.” This event is considered to be high risk, because the amount of energy released and the rate of change of a heavy weight are tremendous. The anti-recoil operation mode of the riser tensioning system aims to perform the riser recoil process in a controlled manner. However, the slow response time of hydro-pneumatic tensioning systems described above can cause poor anti-recoil performance.
First, the anti-recoil system installed on current hydro-pneumatic tensioners is hard to test, because the only way to test is to perform a full length riser recoil. Potential catastrophic consequences may occur during such a test if the anti-recoil system does not work. In that event, it will take weeks of work to recover and is a big loss for both operator and drilling contractor.
Second, riser recoil detection is ineffective on the hydro-pneumatic tensioners, by only relying on sensors installed on vessel, which is highly dependent on vessel motion, and other effects.
Third, the slow response time of the hydraulic and pneumatic equipment during an emergency disconnect situation is insufficient. The slowly-changed large over-pulling force may accelerate the riser to jump out and damage the drill floor and the riser pipes.